Fix #35182: VSE - speed control Trim duration (soft) end reset to 0

Was a regression in rev53509, whic hallowed manual edit of properties
which weren't supposed to be editing manually.

Added the same effects length update as in trnaslation code, so now
updating strip frames from py/interface will keep things consistent.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56467 954f8c5b-7b00-dc11-b283-0030488c597c